Make #wrapper_attributes official and document it

As part of #1838114: Change node form’s vertical tabs into details elements → , the new FAPI property #wrapper_attributes was added to theme_form_element(). However, it was added with the comment // Temporary hack for #type 'item'.

Since #wrapper_attributes was added, it has been used as though it were official throughout core. There are currently 54 instances of the string "#wrapper_attributes" in core, many of which are legitimate uses of the property. The new addition is indeed useful to add things like classes to the wrapper around form elements (especially when working with radio buttons or checkboxes). We should remove the statement of it being a "temporary hack" and make it official. In addition, this new property needs to be documented through a change record.

Beta phase evaluation